Atrial fibrillation is the most common disorder of the heart rhythm requiring treatment with antiarrhythmic drugs, and it accounts for over one-third of all U.S. hospitalizations for arrhythmias. Patients with atrial fibrillation have their usually normal heart rhythm replaced by periods of rapid, irregular heart beating that may cause a sense of pounding in the chest, shortness of breath, chest pain, dizzyness, or loss of consciousness. These periods may last for minutes or hours, or they may last indefinitely unless acute treatment is given by a physician. The most common treatment for symptomatic atrial fibrillation is antiarrhythmic drug therapy. Current drug therapies are limited by lack of efficacy, side effects and concern over safety of these drugs. The purpose of this protocol is to test the efficacy and safety of the antiarrhythmic drug azimilde in mutl-center, placebo-controlled trials.